1. Field of the Invention
The present invention relates to a composite video signal generation method and device, more particularly, it relates to the generation of a color composite video signal conforming to the NTSC (National Television System Committee) system, and to a composite video signal generation method and device wherein digital circuits are preferably employed and are easily integrated.
Generally, when digital color image information processed by using a personal computer or a workstation is displayed on a CRT (cathode ray tube), a method of displaying the image by using the digital signal as is, and another method wherein the digital signal is converted to an analog signal and then displayed on a display device, are used. Particularly, a method of converting the digital color information to a composite video signal conforming to the NTSC system is widely applied when using, e.g., a personal computer, workstation or the like, for an animation process, since the image obtained can be received and displayed by a commercially available television receiver, and thus can be used by the television broadcasting networks.
2. Description of the Related Art
A composite video signal generation device (hereinafter referred to as a color encoder device) which processes analog color composite video signals conforming to the NTSC system converted from digital signals R, G, and B (Red, Green, Blue), receives color image information digital signals R, G, and B from a personal computer, a workstation or the like at three D/A (Digital to Analog) converters, which convert the digital signals R, G, and B to analog signals, in synchronization with a basic clock from a CPU (Central Processing Unit). These analog signals are supplied to a matrix circuit and two chrominance signals (color difference signals) R-Y and B-Y, and a luminance signal Y are calculated. The signal R-Y passes through a lowpass filter and is supplied to a modulation circuit, and the signal B-Y passes through another lowpass filter and is supplied to another modulation circuit. The modulation circuits are supplied with respective, phase-shifted color subcarriers (e.g., at 0 degree and 90 degrees) from an oscillator circuit, which comprises a phase-locked loop circuit. Namely, one of the modulation circuits balance-modulates one of the color subcarriers with the signal R-Y, and another of the modulation circuits balance-modulates the other subcarrier with the signal B-Y. The balance-modulated signals R-Y and B-Y are combined through an addition circuit and the added signal is then combined with the luminance signal Y, which is delayed by a delay line to adjust the arrival time, using another addition circuit, and finally, the signal is output as a composite video signal conforming to the NTSC system.
The above conventional color encoder device, however, converts the digital signals R, G, and B to analog signals and then performs balance-modulation and the like using the thus-obtained analog signals, and therefore, the necessary circuits are not appropriate for circuit integration. Particularly, the analog lowpass filter, the delay line, and the phase-locked loop (PLL) circuit which generates the subcarrier frequency in synchronization with a horizontal synchronizing signal, must be large scale, and thus the production cost is inevitably increased.
Further, since the analog circuit generally cannot avoid characteristic deviations of the devices caused by individual differences in parts characteristics, each device must be adjusted accordingly, and thus the production cost is further increased. Moreover, to obtain stabilized operation over a long term, temperature compensations or changes occurring with the passage of time must be considered, and therefore, a problem of high design costs arises.